From 3c17ad19f0697ffe5ef7438cdafc2d2b7757d8a5 Mon Sep 17 00:00:00 2001 From: John Stultz Date: Wed, 11 Mar 2015 21:16:32 -0700 Subject: [PATCH] timekeeping: Add debugging checks to warn if we see delays Recently there's been requests for better sanity checking in the time code, so that it's more clear when something is going wrong, since timekeeping issues could manifest in a large number of strange ways in various subsystems. Thus, this patch adds some extra infrastructure to add a check to update_wall_time() to print two new warnings: 1) if we see the call delayed beyond the 'max_cycles' overflow point, 2) or if we see the call delayed beyond the clocksource's 'max_idle_ns' value, which is currently 50% of the overflow point. This extra infrastructure is conditional on a new CONFIG_DEBUG_TIMEKEEPING option, also added in this patch - default off. Tested this a bit by halting qemu for specified lengths of time to trigger the warnings. Signed-off-by: John Stultz Cc: Dave Jones Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Prarit Bhargava Cc: Richard Cochran Cc: Stephen Boyd Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1426133800-29329-5-git-send-email-john.stultz@linaro.org [ Improved the changelog and the messages a bit. ] Signed-off-by: Ingo Molnar --- kernel/time/jiffies.c | 1 + kernel/time/timekeeping.c | 28 ++++++++++++++++++++++++++++ lib/Kconfig.debug | 13 +++++++++++++ 3 files changed, 42 insertions(+) diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c index a6a5bf53e86d..7e413902aa6a 100644 --- a/kernel/time/jiffies.c +++ b/kernel/time/jiffies.c @@ -71,6 +71,7 @@ static struct clocksource clocksource_jiffies = { .mask = 0xffffffff, /*32bits*/ .mult = NSEC_PER_JIFFY << JIFFIES_SHIFT, /* details above */ .shift = JIFFIES_SHIFT, + .max_cycles = 10, }; __cacheline_aligned_in_smp DEFINE_SEQLOCK(jiffies_lock); diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 91db94136c10..acf049144cf6 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -118,6 +118,31 @@ static inline void tk_update_sleep_time(struct timekeeper *tk, ktime_t delta) tk->offs_boot = ktime_add(tk->offs_boot, delta); } +#ifdef CONFIG_DEBUG_TIMEKEEPING +static void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) +{ + + cycle_t max_cycles = tk->tkr.clock->max_cycles; + const char *name = tk->tkr.clock->name; + + if (offset > max_cycles) { + printk_deferred("WARNING: timekeeping: Cycle offset (%lld) is larger than allowed by the '%s' clock's max_cycles value (%lld): time overflow\n", + offset, name, max_cycles); + printk_deferred(" timekeeping: Your kernel is sick, but tries to cope\n"); + } else { + if (offset > (max_cycles >> 1)) { + printk_deferred("INFO: timekeeping: Cycle offset (%lld) is larger than the the '%s' clock's 50%% safety margin (%lld)\n", + offset, name, max_cycles >> 1); + printk_deferred(" timekeeping: Your kernel is still fine, but is feeling a bit nervous\n"); + } + } +} +#else +static inline void timekeeping_check_update(struct timekeeper *tk, cycle_t offset) +{ +} +#endif + /** * tk_setup_internals - Set up internals to use clocksource clock. * @@ -1630,6 +1655,9 @@ void update_wall_time(void) if (offset < real_tk->cycle_interval) goto out; + /* Do some additional sanity checking */ + timekeeping_check_update(real_tk, offset); + /* * With NO_HZ we may have to accumulate many cycle_intervals * (think "ticks") worth of time at once. To do this efficiently, diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index c5cefb3c009c..36b6fa88ce5b 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -865,6 +865,19 @@ config SCHED_STACK_END_CHECK data corruption or a sporadic crash at a later stage once the region is examined. The runtime overhead introduced is minimal. +config DEBUG_TIMEKEEPING + bool "Enable extra timekeeping sanity checking" + help + This option will enable additional timekeeping sanity checks + which may be helpful when diagnosing issues where timekeeping + problems are suspected. + + This may include checks in the timekeeping hotpaths, so this + option may have a (very small) performance impact to some + workloads. + + If unsure, say N. + config TIMER_STATS bool "Collect kernel timers statistics" depends on DEBUG_KERNEL && PROC_FS -- 2.20.1